This invention relates to prismatic electrochemical cells and the structure of their housings.
Electrochemical cell shapes are generally classified as either prismatic or cylindrical. Cylindrical cells have, as the name suggests, cylindrical housings. Common examples of cylindrical batteries are standard alkaline sizes AA, AAA, C and D. Prismatic cells have prismatic housing shapes, such as parallelepipeds. Common examples of prismatic cells include standard 12 V car batteries.
Many common types of cells have internal electrode configurations in which the electrodes are in sheet form, with sheets of positive and negative electrode material stacked together and separated by electrically insulating separator sheets. One of the reasons for this face-to-face sheet arrangement is to provide high diffusion area between opposing electrodes.
Ideally, adjacent sheets in the electrode stack remain in intimate "contact" (i.e., very close to each other) over the life of the cell. Suboptimal contact can reduce the overall capacity (total usable energy) of the cell, and can lead to other undesirable effects, such as lithium plating during the charging of Lithium ion (LiION) cells. In cylindrical cells, the stack of electrode and separator sheets is typically rolled up and placed in the can. Tension in the rolled up stack tends to press the stack against the sides of the can (i.e., to move toward its unwound condition) and maintains light pressure between adjacent electrode sheet faces. In prismatic cells, the stacked electrode sheets are either rolled up (as shown in FIG. 3) or folded back and forth (as shown in FIG. 4). The configuration of FIG. 3 may be called wound flat-wrap (WFW), while that of FIG. 4 may be referred to as fan fold or accordion fold.
In longer, thinner prismatic cells (cells with greater proportional distance between the folds or bends of the electrode stack), any tension in the folds or bends of the stack (tending to return the stack to a flat condition) can be insufficient to maintain an appropriate amount of pressure between the sheets along the entire length of the straight portions of the stack without positive compression applied by the side walls of the housing. Such positive compression may be provided by making the housing cavity slightly thinner than the nominal overall stack thickness, or by inserting a spring member (such as a leaf spring) within the housing to bias the straight portions of the stack together.
Positive compression against the straight portions of the electrode stack generally tends to bow the broad sides of the housing outward. This can add to the effect of internal cavity pressure to result in undesirable distortion of the outer cell shape if the sides of the housing are not constructed to adequately resist the applied bending moments. Of course, the stiffness of the housing side walls can be increased by increasing their thickness, but typically at a penalty of increased weight and, for a standard size cell, at a loss of internal volume. Internal springs can also occupy internal cell volume that may otherwise be available for active material.